This invention relates to a fluid-bed catalytic process for upgrading olefinic light gas feedstock (termed "light gas" for brevity herein) containing lower, particularly C.sub.3 -C.sub.5, olefins (alkenes) and paraffins (alkanes). The olefins are converted to heavier hydrocarbons ("heavies") in a single-zone fluid-bed reactor, operating at pressure and temperature conditions at which it is critical that there be no liquid phase present. These operating conditions are referred to herein as P.sub.max and T.sub.max. Such conditions prevail at near-critical and super-critical pressure and/or temperature in a super-dense phase turbulent regime. By "near-critical" we refer to a pressure of at least 2857 kPa (400 psig), and a temperature of at least 204.degree. C. (400.degree. F.) which is always above the critical temperature of the feed. In other words, the reactor converts light gas to heavies in a single zone of turbulent regime, operating at a pressure and temperature outside a tightly circumscribed region of pressure and temperature ("critical P & T region") which region lies near, or above the apex of a phase diagram defining the critical point (P.sub.cr, T.sub.cr) of the mixture of hydrocarbons in the reactor. We know of no other process which can convert olefins to distillate or lube oil in high yield, in a single zone.
Moreover, we accomplish this with an unexpected degree of flexibility. The operating pressure and temperature conditions may be supercritical (that is, both are above P.sub.cr, T.sub.cr of the mixture); or, only one or the other may be below P.sub.cr, T.sub.cr ; or, both may be below P.sub.cr, T.sub.cr ; provided they are not in the critical P & T region so that no liquid can form. Operation at precisely P.sub.cr, T.sub.cr conditions, or too close to them (within the critical P & T region), involves too high a risk of formation of a liquid phase, and is therefore avoided. At, or above P.sub.max and T.sub.max, under near-critical or super-critical conditions, the entire bed is in a fluid phase in which the solid acts as catalyst and heat transfer sink. In this process, we regard the super-dense phase as being neither gas nor liquid, but for convenience and familiarity, we treat the oligomerization reaction as being a gas/gas reaction.
More particularly, the invention provides a continuous process for oligomerizing light gas containing propene, butenes and pentenes, preferably in the absence of added hydrogen, to a C.sub.10.sup.+ rich hydrocarbon stream, in either of two operating modes. In one mode the reactor is operated at relatively low pressure in the range from about 2857 kPa to about 10436 kPa (400 psig-1500 psig), and relatively high temperature in the range from 260.degree. C. to about 371.degree. C. (500.degree. F. -700.degree. F.), referred to as the "distillate mode"; in the other mode the reactor is operated at relatively high pressure in the range from about 5270 kPa to about 13881 kPa (500 psig-2000 psig), and relatively low temperature in the range from 204 C. to about 315.degree. C. (400.degree. F.-600.degree. F.), referred to as the "lubes mode". Even higher pressures, as high as 20821 kPa (3000 psig) may be used if the economics of operating at such high pressure can be justified by the lube "make". By "distillate" we refer to C.sub.10.sup.+ hydrocarbons boiling in the range from about 138.degree. C. to about 349.degree. C. (280.degree. F.-660.degree. F.); by "lubes" we refer to high-boiling hydrocarbons having a viscosity in the range from 10 cp to about 100 cp, measured at 100.degree. C. The particular operational mode chosen depends upon which particular boiling range of oligomerized product is desired, though in either case a minor amount of C.sub.5.sup.+ gasoline range hydrocarbons may also be formed. When this occurs, the gasoline, typically not a desired product in our process, is recycled to yield the desired distillate or lubes product. Light gas containing a substantial, preferably a major portion, typically more than 75% of combined propene and butenes, is a particularly well-suited feed to the reactor.
Developments in fluid-bed catalytic processes using a wide variety of zeolite catalysts have spurred interest in commercializing the conversion of olefinic feedstocks for producing C.sub.5.sup.30 gasoline, diesel fuel, etc. In addition to the discovery that the intrinsic oligomerization reactions are promoted by ZSM-5 type zeolite catalysts, several discoveries relating to implementing the reactions in an apt reactor environment, have contributed to the commercial success of current industrial processes. These are environmentally acceptable processes for utilizing feedstocks that contain lower olefins, especially C.sub.3 -C.sub.5 alkenes, though some ethene (ethylene), and some olefins and paraffins heavier than C.sub.5 may also be present.
Of particular interest is that the ZSM-5 type catalyst used under our severe process conditions does not appear to suffer from a sensitivity (poisoning) to basic nitrogen-containing organic compounds such as alkylamines ( e.g. diethylamine), or, to oxygenated compounds such as ketones, a proclivity which is characteristic of the catalyst under the process conditions of prior art olefin oligomerization processes. Such processes require the addition of hydrogen as a preventative antidote. It will be recognized that alkylamines are used in treating light gas streams, and ketones are typically present in Fischer Tropsch-derived light ends streams, both of which streams are particularly well-suited for upgrading by oligomerization. Though our process is not adversely affected by the presence of hydrogen, there is no readily discernible economic incentive for using it in our single stage reactor, and we prefer not to do so.
Conversion of C.sub.3 -C.sub.5 alkenes and alkanes to produce aromatics-rich liquid hydrocarbon products were found by Cattanach (U.S. Pat. No. 3,760,024) and Yan et al (U.S. Pat. No. 3,845,150) to be effective processes using the ZSM-5 type zeolite catalysts. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al have also contributed to the understanding of catalytic olefin upgrading techniques and have contributed improved processes as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The '062 patent discloses conversion of olefins to gasoline or distillate in the range from 190.degree.-315.degree. C. and 42-70 atm; and this, and the '640 and '992 disclosures are incorporated by reference thereto as if fully set forth herein.
The prior art processes relate to the conversion of lower olefins, especially propene and butenes, over ZSM-5 and HZSM-5, at moderately elevated temperatures and pressures. The sought-after conversion products are liquid fuels, especially the C.sub.6.sup.30 aliphatic and aromatic hydrocarbons. It is known that the product distribution may be tailored by controlling process conditions, such as temperature, pressure and space velocity. Gasoline (C.sub.6-10) is readily formed at elevated temperature (preferably about 400.degree. C.) and pressure from ambient to about 2900 kPa (420 psia), preferably about 250 to 1450 kPa (36 to 210 psia). Olefinic gasoline can be produced in good yield and may be recovered as a product or fed to a low severity, high pressure reactor system for further conversion to heavier distillate-range products. Distillate mode operation can be employed to maximize production of C.sub.10.sup.+ aliphatics by reacting the lower and intermediate olefins at high pressure and moderate temperature. Operating details for typical "MOGD" (for Mobil Olefin to Gasoline & Distillate) oligomerization units are disclosed in U.S. Pat. Nos. 4,456,779 and 4,497,968 (Owen et al); 4,433,185 (Tabak); and Ser. No. 006,407 (Avidan et al) filed Jan. 23, 1987issued as U.S. Pat. No. 4,746,762. Because our process is preferably operated for the production of distillate and lubes it is referred to as the "MODL" (for Mobil Olefin to Distillate & Lubes) process.
Lower olefin feedstocks containing C.sub.2 -C.sub.6 alkenes may be converted selectively either to a higher or lower boiling range product by varying pressure, temperature, the type of catalyst, and the mass flow through the reactor. Despite the very high-pressure and temperature used in the reaction of our process, the difficulty of converting ethene is not overcome, and, unlike the foregoing '407 Avidan et al process, our process is not particularly directed to the conversion of a major amount of ethene, if it is present in the feed. However our process may oligomerize a good portion of ethene. It is well known that ethene conversion increases as temperature increases in the range from about 204.degree. C. to 316.degree. C. (400.degree.-600.degree. F.), but the strictures of operation in the prior art fluid-bed processes demand a catalyst cycle time which does not permit good conversion in this temperature range at conventional fluid-bed pressures. Because our process, in its high temperature mode, can operate continuously at the high end of the foregoing temperature range at near-critical or super-critical pressures, it favors oligomerization of ethene, but because of the higher pressure and bed density than in the Avidan '407 process, oligomerization of higher alkenes than ethene is far more highly favored.
To date, in a refinery for crude petroleum, very high pressure reactors (at least 400 psig) in which there is a deliberate effort made to provide a thoroughly mixed catalyst in the fluid phase, was limited to the multi-phase contacting of liquid-solids-gas systems, such as in the hydrogenation of liquids in the presence of catalyst, as for example in the hydrogenation of heavy residuum hydrocarbons for the purposes of hydrodesulfurization, hydrocracking or similar processes, exemplified by that disclosed in U.S. Pat. No. 3,363,992 to Chervenak.
We have now found that C.sub.3 -C.sub.4 -rich and higher olefins may be selectively upgraded to normally liquid hydrocarbons in either the distillate or lubes ranges by catalytic conversion in a turbulent fluidized bed of solid acid zeolite catalyst with an alpha (activity) in a broad range from about 2 to 100,operating at or above P.sub.max and T.sub.max, in the super-dense phase, in the absence of added hydrogen, in a single pass, or with recycle of undesired oligomerized product.
However, the most important advantage is the close temperature control afforded by operation of a fluid-bed in the turbulent regime (referred to as a "turbulent bed"). An essentially uniform conversion temperature may be maintained (often with closer than .+-.5.degree. C. tolerance). Except for a small zone adjacent the bottom gas inlet, the midpoint measurement is representative of the entire bed, due to the thorough mixing achieved. Nothing in the prior art teaches how to operate a super-dense phase fluid-bed in the turbulent regime, or what the effects of doing so might be. Nor is there any suggestion as to how to predict the minimum and complete fluidization velocities, U.sub.mf and U.sub.cf respectively, or the minimum bubbling velocity U.sub.mb, at or above P.sub.max and T.sub.max.
The foregoing notwithstanding, the simple fact was that at a pressure sufficiently high to produce a super-dense phase in the bed, it was unlikely that there would be the requisite amount of backmixing deemed an essential characteristic of a turbulent bed of an aeratable (group A) powder. Much backmixing of gas in the dense phase occurs, and gas exchange between bubbles and the dense phase is high, due to splitting and recoalescence, which is why the turbulent regime in a dense phase bubbling bed provides a region of backmixing (see E. R. Gilliland and E. A. Mason, Ind. Eng. Chem., 41, 1191, 1949). In a super-dense phase bed, there is no experimental evidence to expect that bubbles would drag solids upwards, and that a backflow of solids elsewhere would carry interstitial gas downward. Since, at very high pressure, the bubbles are not only minute, but have minimal vertical movement, it appeared most unlikely that operation at near-critical or super-critical pressure would provide the necessary backmixing.
Moreover, in turbulent beds, fluidization is better at a higher fluidizing gas velocity, and with a higher level of the finer sizes of catalyst (see R. M. Braca and A. A. Fried, in Fluidization, D. F. Othmer, Ed. (Reinhold, New York, 1956), pp. 117-138; W. W. Kraft, W. Ulrich, W. O'Connor, ibid., pp. 184-211). This requires a significant amount of fines, from about 10 to 25 % by weight (% by weight) having a particle size less than 32 microns. In a super-dense, turbulent bed, how and why would one expect to confine such a relatively large amount of fines, assuming they were desirable, if not essential, for the operation of the bed?
U.S. Pat. Nos. 4,417,086 and 4,417,087 to Miller teach a two-zone reactor operating in the transport mode where the relative superficial gas velocity is greater than the terminal velocity in free fall. Though the operation of a fluid-bed is illustrated (example 2 in each of the '086 and '087 patents) note that no operating pressure is state din the former, and that operating pressure in the latter is 10 psig (24.7 psia, 170 kPa). The general disclosure that the processes may be operated at a pressure in the range from subatmospheric to several hundred atmospheres, but preferably 10 bar or less, and most preferably 0 to 6 bar, (see middle of col 6 in '086, and, near top of col 5 in '087) is not so ingenuous as to be meant to apply equally to the fixed bed (example 1 of '086 and '087, each illustrates 34.5 bar, 500 psi) and the 170 kPa fluid-bed.
Another incidental disclosure of operation of a fixed bed of a zeolite, particularly a large pore zeolite having pore dimensions greater than about 6 A (angstroms), at high pressure, up to 13,780 kPa (2000 psig) and temperature, up to 399.degree. C. (750.degree. F. ) for the catalytic conversion of olefins to heavies, is found in U.S. Pat. No. 4,430,516 to Pierre et al. But they did not state the obvious, namely, that the conditions they specified, applied to a fixed bed reactor; nor could they have, because at that time, a fluid-bed in a dense phase turbulent regime, let alone a superdense one at near-critical or super-critical conditions, had simply never been considered, let alone operated.
Moreover, if denser fluidization regimes are viewed as comprising systems in which transient clusters of relatively large, dense aggregates of particles are dispersed in a dilute continuum of sparsely distributed, smaller clusters, how would a superdense regime be viewed? How applicable would the empirical Richardson-Zaki equation (see J. F. Richardson, and W. F. Zaki, Trans. Inst. Chem. Eng. 32, 35, 1954) be under near-critical or supercritical conditions? Finally, apart from the mechanics of operating the super-dense fluid-bed, how would the catalytic activity of porous crystalline aluminasilicate catalysts, (group A powders, see A. M. Squires, M. Kwauk and A. A. Avidan Science, 230, 1329-1337, 1985) be affected by conditions in the super-dense bed? Since fluid-beds are more competitive in larger sizes, and scale-up of group A fluid-beds is notoriously difficult when high conversions of reactants are desired, how would one expect to provide an operable super-dense fluid-bed? How would one expect to cope with the reality that near-critical and super-critical conditions for the product would be quite different from those for the light gas feed, or recognize the significance of the difference?
Thus to date, a high pressure zeolite fluid-bed process was limited to a pressure of 2500 kPa (363 psia). For example, in the '407 Avidan process, the ethene-containing C+olefinic feedstock is converted in a catalytic reactor operating under elevated pressure (410 to 2500 kPa) and temperature (315 to 510.degree. C.) to convert at least 70% of feedstock ethene in the light gas into C.sub.8 .sup.+ hydrocarbons rich in gasoline-range olefins and aromatics. But the process conditions of our invention are substantially different from those for the '407 process, and though our process favors conversion of some ethene, as will be explained hereinafter, our process is not directed to the conversion of a major proportion of ethene, yet is quite suddenly highly effective with C.sub.3.sup.=+ olefins.